Some simple routines can be used to discover general characteristics of the display.
These shall be described in some detail as an introduction to some X concepts.

int DefaultScreen(display) /* This returns the default screen number; the only */
/* one you're likely to use */
int ScreenCount(display) /* This returns the number of possible screens */
int DefaultDepth() /* A graphics display can be thought of as overlying
bitplanes where only one bit per plane affects the
color of a pixel. This routine returns the number
of planes used */
unsigned long AllPlanes() /* The bits in the returned long
represent the planes used */
int DisplayPlanes(display, screen_number) /* returns the number of possible planes*/
Colormap DefaultColormap(display, screen_number) /* Information on the current
colors is stored in a table called a colormap */
int DisplayCells(display, screen_number) /* returns number of entries in the
default colormap */
unsigned long BlackPixel(display, screen_number) /* There are permanently */
unsigned long WhitePixel(display, screen_number) /* allocated entries in
the default colormap for black and white. These routines return
longs that are used to reference these colors */
GC DefaultGC(display, screen_number) /* Graphics commands get details about
the current foreground color, etc, from a Graphics Context structure.
This routine returns the default one */
Window DefaultRootWindow(display) /* This returns the initial window that
covers the screen */
Visual *DefaultVisual(display,screen_number) /* So that various kinds of hardware
can be supported `invisibly', some hardware-specific information
is stored in the visual structure*/
int DisplayHeight(display,screen_number) /* height of display in pixels */
int DisplayWidth(display,screen_number) /* width of display in pixels */